A hydraulically driven type of travelling apparatus is designed to be driven with a pair of hydraulic motors. The hydraulic motors are disposed at right and left hand sides for right and left hand side driving wheels, respectively. In addition, the hydraulic motors are for moving a vehicle. The right and left hand side traveling purpose hydraulic motors are supplied with a pressurized discharge fluid from a hydraulic pump, by right and left hand side directional control valves, respectively.
A travelling control circuit for such a hydraulically driven type of travelling apparatus is known in the prior art, as shown in FIG. 1, in which a single hydraulic pump is provided.
In such a travelling control apparatus, a hydraulic pump 1 has a discharge path 2 that is provided with a left hand side directional control valve 3 and a right hand side directional control valve 4 which are disposed in parallel to each other. A pair of first circuits 7 and a pair of second circuits 8, arranged at the left and right hand side, couple the left and right hand side directional control valves 3 and 4 with left and right hand side hydraulic motors 5 and 6. The hydraulic motors 5 and 6 are used for moving the machine. A pressure compensating valve 9 is provided in each of the first circuits 7 and each of the second circuits 8 to respond to the higher of load pressures of the left and right hand side hydraulic motors 5 and 6.
More specifically, each of the pressure compensating valves 9 is pushed in a direction to disconnect an inlet port 12 from an outlet port 13 by a spring 10 and by a maximum load pressure that acts on a first pressure receiving portion 11. Furthermore, each of the pressure compensating valves 9 may also be pushed in a direction to place the inlet port 12 into communication with the outlet port 13 under its own load pressure that acts on a second pressure receiving portion 14. Hence, the area of opening between the inlet port 12 and the outlet port 13 of each of the pressure compensating valves 9 will decrease as the difference between the maximum load pressure and its own load pressure is increased, thus compensating for a pressure in the inlet side.
Since this causes the pressurized discharge fluid from a single hydraulic pump to be supplied simultaneously to the left and right hand side hydraulic motors 5 and 6 while also compensating for a pressure with the pressure compensating valves 9, a pressurized discharge fluid can be supplied to the left hand side hydraulic motor 5 and the right hand side hydraulic motor 6 even if the left and right hand side traveling purpose hydraulic motors 5 and 6 have different load pressures.
At this point, it should be noted that a first auxiliary circuit 15 and a second auxiliary circuit 16 are each designed to control communication between the outlet port side of the pressure compensating valve 9 and a reservoir port of the left and right hand side directional control valves 3 and 4. Also, it should be noted that a capacity control unit la is provided for controlling the capacity of the hydraulic pump 1 in accordance with a load pressure.
Assume now that the foregoing travelling control arrangement is adopted. Then, if the left and right hand directional control valves 3 and 4 are switched to the first position A as shown in FIG. 2, to travel linearly, and if the supply flows to the left and right hand travelling purpose hydraulic motors 5 and 6 are different, the apparatus will travel in a curve. Then, as shown in FIG. 1. it is configured that the left and right hand side first circuits 7 and the left and right hand side second circuits 8 are each short circuited with a short circuit path 17. Each short circuit path 17 is provided with a travelling communicating path 18. The travelling communicating paths have communicating positions a which supply an identical flow quantity to the left hand side and the right hand side travelling purpose hydraulic motors 5 and 6 at the time of linear movement. When the travelling hydraulic motors 5 and 6 are turning leftwards and rightwards the travelling communicating valves 18 take the blocking positions.
In such a travelling control circuit, the pair of travelling communicating valves are required and costs are increased.
In such a travelling control circuit, it should also be noted that it is necessary for each of the travelling communicating valves 18 and 18 to switch between the communicating position a and the blocking position b synchronously with the switching operation of the directional control valves 3 and 4.
For this reason, it has been suggested to control the traveling communication valve 18 by a pressure acting on its pressure receiving portion 18a and also to couple a pilot pressure supply circuit 19 to the pressure receiving portion 18a and to the respective pressure receiving portions 3a and 4a of the left and right hand side directional control valve 3 and 4. Accordingly, in addition to the pair of short circuit paths 17 and 17, the four pilot pressure supply circuits 19 are also needed thus complicating the entire circuit.
In view of the foregoing problems, it is the object of the present invention to provide a travelling control circuit for a hydraulically driven type of travelling apparatus where the two travelling communication valves are unnecessary, thus decreasing costs, and where a pilot pressure fluid supply is unnecessary, thus simplifying the entire circuit.